Swimmers were warned to stay at least 300 feet away from seals. Sharks have been more visible along Cape Cod this summer with numerous sightings. Experts blame a drastic increase in the area's seal population on which sharks feed.

"The elbow of the cape has these large, dense concentrations of gray seals now, and these white sharks go to the area to feed," said Greg Skomal, a senior biologist at the Massachusetts Division of Marine Fisheries. "Because the seals are so abundant, now the white sharks are paying more attention."

A man was attacked in July by what is believed to be a great white shark in the waters off Ballston Beach in Truro, Mass. Police said Chris Myers was bit in both legs below the knees in possibly one single, crushing blow.

"I've been swimming at that beach since I was a little kid, and no one in recent memory has ever had a shark attack, let alone by a great white, which they are saying they think it was," Myers told "Good Morning America after the suspected shark attack. "Maybe people need to be a little more careful."

Three weeks before that incident, a great white shark was spotted trailing a kayaker at Nauset Beach, about 25 miles south of Ballston Beach.